(1) Field of the Invention
The present invention relates to methods of fabricating semiconductor devices and more specifically to a method used to reduce a short channel effect resulting from segregation of boron into surrounding silicon oxide regions.
(2) Description of Prior Art
The ability to fabricate semiconductor devices in a silicon on insulator (SOI) layer has allowed specific performance degrading capacitances to be reduced. However the presence of an additional silicon oxide region, the insulator component of the SOI layer, presents another region in which boron can be removed from a device region, segregated to that silicon oxide area. Since boron is used for threshold adjustment of N channel metal oxide semiconductor (NMOS) devices the unwanted removal of boron from the channel region can result in unwanted reductions of NMOS threshold voltage. With decreasing channel length designs boron segregation, specifically at the perimeter of a narrow channel length NMOS device, can result in an unwanted short channel phenomena.
The present invention will describe methods of reducing the risk of a short channel phenomena with a first embodiment featuring creation of boron doped regions in silicon oxide regions located underlying the NMOS channel region, with the boron doped oxide region now satisfying the silicon oxide regions affinity for boron. A second embodiment will describe formation of barrier layers on surfaces of shallow trench isolation regions filled with silicon oxide regions, with the barrier layers preventing boron from being segregated from an adjacent NMOS channel region to adjacent silicon oxide region. Prior art such as Sakaguchi et al in U.S. Pat. No. 6,613,678 B1, Xiang in U.S. Pat. No. 6,410,938 B1, Iwamatu et al in U.S. Pat. No. 5,656,537, and Iwamatsu et al in U.S. Pat. No. 5,910,672, describe methods of forming devices in or on SOI layers. None of the above prior art however describe the methods employed in present invention in which silicon oxide affinity for boron is decreased, or wherein boron is blocked from reaching the silicon oxide regions, allowing the desired level of boron in the NMOS channel region, to remain undisturbed.